Abstract: A heating appliance having a housing which houses a water and air heating system. The water heating system includes a heat source and a first heat exchanger for transferring heat to a working fluid passing through the housing. The air heating system includes a water-to-air heat exchanger in communication with the working fluid and a source of air, for heating air using the working fluid. The housing defines an upper compartment for the heat source and first heat exchanger, and a lower compartment for the water-to-air heat exchanger.

Abstract: A condensing heat exchanger of the kind having a primary heat exchanger and a secondary heat exchanger connected downstream of and in series with the primary heat exchanger. The secondary heat exchanger is arranged to at least partially condense combustion gases discharged from the primary heat exchanger. The primary heat exchanger comprising a drum having a longitudinal axis and the secondary heat exchanger comprising a plurality of small diameter tubes arranged by the side of the drum and extending parallel with the longitudinal axis of the drum.

Abstract: An electrical circuit having three states depending upon the magnitude of an electrical signal coupled, in operation, to an input thereof, the circuit having a first stable state when the said magnitude is less than a first, predetermined value, a second, stable state when the said magnitude is greater than a second, predetermined value higher than the first said predetermined value, and a third, unstable state when said magnitude is equal to or between said first and second predetermined values. In a development of the invention, the circuit is incorporated in a control apparatus for a ducted, warm-air, central-heating system in which the gas supply to a burner is supplied continuously when the temperature of a space to be heated is below a predetermined temperature, is cut-off when the temperature of the space is above another, higher predetermined temperature, and is switched on and off at a rate dependent upon the magnitude of the temperature when it is equal to or between the two values.